1. Field of the Invention
The present invention relates to a numerical controller satisfying both of avoidance of overheating of a motor due to a continuous operation of a program and maintenance of machining quality.
2. Description of the Related Art
A machine tool is a machine capable of machining desired workpieces by operating according to a machining program created in advance. Accordingly, a plurality of pieces of desired workpieces can be machined by operating one machining program continuously and repeatedly.
A drive motor for machine tools has, as shown in FIG. 6, a continuous rating output and a maximum output based on characteristics of the motor defined therefor. Such a drive motor is normally used within a range of the continuous rating output, but may also be used by exceeding the continuous rating output depending on an operation pattern when, for example, high-load machining is temporarily performed. This is done in the context of being able to keep costs lower than the introduction of a new machine tool of a larger continuous rating load to temporarily perform high-load machining.
However, if a machining program having a motor operation pattern exceeding the continuous rating load of a drive motor is continuously operated, the motor temperature rises gradually.
FIG. 7 shows a graph showing a case where a machining program having a motor operation pattern exceeding the continuous rating load of a drive motor is continuously executed, with the vertical axis representing the motor temperature and the horizontal axis representing time. As is understood from the graph in FIG. 7, if a machining operation is performed according to a machining program having a motor operation pattern exceeding the continuous rating load, the temperature of the motor does not fall sufficiently even though the machining operation is completed and the temperature of the motor rises gradually as repetitive machining is continued, and finally, the temperature of the motor exceeds an overheating temperature and overheating occurs so that the machine stops.
To solve such a problem, for example, JP 2000-271836 A proposes a control method of a machine tool that exercises control such that overheating is prevented from arising by, when a machining program is continuously executed, as shown in FIG. 8, changing a cutting feed speed override and a time constant for acceleration/deceleration momentarily based on machine states such as the temperature of the motor to automatically adjust actual machining operation content.
According to the above control method, however, a machining operation that is technically different from operation content instructed by an NC program is performed by a machine tool as shown in FIG. 9 by changing the cutting feed speed override or the time constant for acceleration/deceleration momentarily in accordance with the heating state and the temperature state of the machine too, with the result that the quality of each of machined workpieces varies. That is, there is a problem that it is impossible to satisfy both of avoidance of overheating of a motor due to a continuous operation of a program and maintenance of machining quality.